Why Am I Having Premenstrual Pain After Putting Mirena IUD?

I have had a Mirena IUD fitted 3 days ago and am in so much pain. It feels like mentrual cramping but so much more severe. I had it fitted mid-cycle and that bleeding has stopped. The pain isn't constant but it hits sporadically and is so extreme I can't breath or move. Is this normal and will it ease anytime soon? I have PCOS and extremely painful, heavy periods so had the Mirena to try and make that better.